This invention relates to devices for cutting materials such as slabs of plastic foam by use of a cutting wire.
In the prior art, such as described in U.S. Pat. Nos. 2,646,494 or 3,884,104 devices are presented for cutting patterns in slabs of rigid plastic foam, such as styrofoam. These devices comprise a flat horizontal table and a cutting wire which is maintained perpendicular to the table and under tension between an attachement point under the table level and an attachement point on a beam extending over the table. The wire is heated by an electric current, and the operator can cut a shape out of a styrofoam slab by manually sliding and driving the slab on the table, and visually controlling the path of the heated wire into the slab. to obtain more precise shapes, the operator must fix a template at the desired shape on one face of the slab and slide the slab on the flat table while making sure that the wire is maintained in contact with the edge of the template and cuts its way around the template. Large volumes can be obtained by piling a number of styrofoam shapes obtained from the device, but to achieve a volume with a smooth surface this technique requires extensive cutting and sanding to eliminate the steps resulting on the surface of the volume from each slab, because the heated wire could not be tilted to appropriate angles tangent to the desired surface during the cutting of the slabs. Thus the final desired volume can be obtained from the stepped volume only by extensive sanding.
For this reason, in the creation of large volumes, the alternative approach of using only a hand-held heated wire maintained in tension by a bow-like frame is still most often preferred. In the surfboard, windsurfing and model-making industries, shapers have been using such a type of hand-held tool for directly sculpting the desired shapes and volumes from large blocks of plastic foam. The results are checked during the operation by repeated applications of control templates at the surface of the foam volume. A large percentage of the original foam block is usually wasted. The final approach to the desired shape relies on extensive sanding and filling. In an alternative method also making use of a hand-held heated wire, but applying the slice-by-slice building of a large volume and the use of commercial slabs of plastic foam, templates are pinned to both faces of the plastic foam slab, and the heated wire is manually driven along both templates. The process is repeated for each slice element of the volume, and the slices are then glued together to compose the whole volume. The obtained volume is much closer to the final desired volume than by use of the cutting device described by U.S. Pat. Nos. 2,646,494 or 3,884,104; but it is a tedious and complex task to define and cut all the templates and it is difficult to coordinate by hand the travel of the cutting wire along both templates at a uniform rate.
Therefore the main object of the present invention is to mechanize these manual techniques in order to make them more precise and less time-consuming to operators in the model-making, mold-making, aeronautical or watercraft industries. The resulting device is related to the devices of the prior art described in the beginning of this paragraph, in the sense that slabs of plastic foam are moved in the plane of one of their faces while being cut by a wire stretched between a point under this plane and a point on a structure extending over said plane. However this invention extends this technique far beyond its previous limitations, by allowing an automatic rotation of the plastic foam, as well as an automatic translation and tilting of the cutting wire, these motions being calculated and monitored by a microcomputer from the drawings made on the screen of this microcomputer. In this sense, the present invention also relates to computer-controlled cutting machines such as presented in U.S. Pat. No. 4,393,450, which describes a lathe-type arrangement for generating 3-D models out of plastic foam, wherein a cutting wire perpendicular to the axis of rotation of the plastic foam is mounted on the head stock of the lathe. However, the divergences of objectives of this invention with respect to this prior art result in specific mechanical and kinematic features, as well as distinctive advantages. The present invention is aimed at cutting and shaping single slabs as elements of large volumes and therefore provides the means for creating volumes which are very large compared to the size of the machine. The specific kinematics of the cutting wire in this invention, combined with the technique of building volumes out of slabs, enable the operator of the present device to create volumes with concavities which could not be obtained by the system described in U.S. Pat. No. 4,393,450, because the frame parts holding the wire ends in that system would hit the plastic foam in an attempt to cut such concavities.